REVERSIBILITY OF THE COLLOIDAL CONDITION OF SOILS 481 
The meaning of the word reverszbility as above interpreted is too narrow 
when applied to soils. Ehrenberg (1915) states that the meaning is not 
so much erroneous as of too little extension. In dealing with soils from 
the agricultural standpoint the cases in which a hydrosol is possible are 
the exception, not the rule. Such cases would be the flooding of rice 
fields, cranberry bogs, and other soils the crops or the culture of which 
"equire an excess of water at times. Reversibility of soil colloids in the 
‘arrow sense above described would be of more or less importance in the 
case of a temporary excess of water, as in rains and in the application of 
irrigation waters, especially by the fiooding methods. From the stand- 
point of the geological formation of alluvial, lacustrine, and marine soils, 
this sort of reversibility would be of considerable importance. 
However, in considering the reversibility of the colloidal state of the 
great masses of agricultural soils, one’s concern is more with the change 
from the gel to the non-gel form and vice versa. The reversibility studies 
of Van Bemmelen with various oxides dealt with the gel and non-gel 
forms, and the word reversibility is used by some writers in referring to 
those changes. It is a word which is convenient to use and is expressive. 
In a consideration of the reversibility of the colloidal condition of soils, 
it seems that the indirect cases which occur thru chemical and biological 
actions should be included, such as the hydration of iron salts, the action 
of bacteria, and the growth of lower forms of plant life. {t will be seen 
in the presentation of results that some of these actions are considerably 
affected by changes in the moisture content of the soil. Reversibility of 
the colloidal state of soils should include, therefere, all cases brought 
about by physical, chemical, and biological agencies or a combination 
of these. In fact, most of the changes of the colloidal states of soils are 
probably due to a combination of forces. These changes may be repre- 
se’:.ed as follows: 
{ Sol 
Non-colloidal state [~~ Colloidal state; y 
| Gel 
COLLOIDAL MATERIALS IN SOILS 
Way (1854) reported deposits of soluble or gejatinous silica in the lower 
beds of the chalk formation. The silica, soluble in alkalies, in the samples 
examined ranged from 5 to 72 per cent. 
